Authors: Andreea Marza-Ene, Liviu Matenco, Hannah Pomella, Vlad Victor Ene, Ioan Munteanu, Anneke Royakkers
Summary:
In the lifetime of an orogenic cycle (mountain building), an episode of collapse called „orogenic extension” takes place during or after the main compressional stage and it is explained in the literature through various mechanisms. Most commonly, orogenic extension is represented by the rise of the lower crust to shallower depths or even to the surface. In an extensional tectonic regime, the contrast in metamorphic grade between the exhumed lower crust and the host rocks is reffered to by geoscientists as metamorphic core complexes. If these complexes have later been affected by subsequent deformations or overprinted by magmatism, distinguishing between different deformation stages becomes critical for establishing a coherent evolution of an orogen. This is usually resolved through detailed structural and kinematic analysis. One of the few places where European Hercynian metamorphic core complexes (of the orogen-parallel extensional dome type) can be studied is in North Dobrogea area, in Romania.
The study aims to explain the structural evolution of the Macin unit of the North Dobrogea orogen from the burial stage and associated prograde metamorphism to the exhumation stage with its related structures and mineral transformations. To separate between the deformation stages, outcrop observations and thin sections analysis were integrated. From a chronological perspective, low-temperature fission track thermochronology was used to date the exhumation of the metamorphic rocks.
Our structural observations indicate the presence of only two mineral lineations, each characteristic of a distinct mineral assemblage. The first lineation defined by prograde minerals as amphibole and plagioclase, characteristic of prograde metamorphism, whereas the second lineation is defined by oriented chlorite, indicative of retrograde metamorphism. This implies a single orogenic cycle with one burial stage and one exhumation stage, rather than multiple orogenic cycles as proposed in earlier models.
The results of fission track analysis on apatite and zircon do not record the movement along the detachment (i.e. the exhumation of the metamorphic rocks from greater depths into the Paleozoic cover). Instead, the recorded ages date the later thrusting of the entire Macin unit, implying that exhumation occured prior to Cimmerian thrusting (Late Jurassic – Early Cretaceous, 145 to 78 Ma).
This study employs a modern approach, combining kinematic analysis and low temperature fission track thermochronology, to propose a new structural model for the „oldest” orogen in Romania. This has a direct impact on the „Geology of Romania” course taught in geology-focused faculties in Cluj, Iasi, Sibiu and Petrosani.
For the general public, these results contribute to geological knowledge that can be integrated into interpretive panels across Macin Mountains National Park and in its visitor center from Greci.